Fix cubic ReDoS in fenced code and references

Two regular expressions were vulerable to Regular Expression Denial of
Service (ReDoS).

Crafted strings containing a long sequence of spaces could cause Denial
of Service by making markdown take a long time to process.
This represents a vulnerability when untrusted user input is processed
with the markdown package.

ReferencesProcessor:

https://github.com/Python-Markdown/markdown/blob/4acb949256adc535d6e6cd8/markdown/blockprocessors.py#L559-L563

e.g.:

```python
import markdown
markdown.markdown('[]:0' + ' ' * 4321 + '0')
```

FencedBlockPreprocessor (requires fenced_code extension):

https://github.com/Python-Markdown/markdown/blob/a11431539d08e14b0bd821c/markdown/extensions/fenced_code.py#L43-L54

e.g.:

```python
import markdown
markdown.markdown('```' + ' ' * 4321, extensions=['fenced_code'])
```

Both regular expressions had cubic worst-case complexity, so doubling
the number of spaces made processing take 8 times as long.
The cubic behaviour can be seen as follows:

```
$ time python -c "import markdown; markdown.markdown('[]:0' + ' ' * 1000 + '0')"
python -c "import markdown; markdown.markdown('[]:0' + ' ' * 1000 + '0')"  1.25s user 0.02s system 99% cpu 1.271 total
$ time python -c "import markdown; markdown.markdown('[]:0' + ' ' * 2000 + '0')"
python -c "import markdown; markdown.markdown('[]:0' + ' ' * 2000 + '0')"  9.01s user 0.02s system 99% cpu 9.040 total
$ time python -c "import markdown; markdown.markdown('[]:0' + ' ' * 4000 + '0')"
python -c "import markdown; markdown.markdown('[]:0' + ' ' * 4000 + '0')"  74.86s user 0.27s system 99% cpu 1:15.38 total
```

Both regexes had three `[ ]*` groups separated by optional groups, in
effect making the regex `[ ]*[ ]*[ ]*`.

Discovered using [regexploit](https://github.com/doyensec/regexploit).

markdown/blockprocessors.py

@@ -559,7 +559,7 @@ def run(self, parent, blocks):
 class ReferenceProcessor(BlockProcessor):
     """ Process link references. """
     RE = re.compile(
-        r'^[ ]{0,3}\[([^\]]*)\]:[ ]*\n?[ ]*([^\s]+)[ ]*\n?[ ]*((["\'])(.*)\4|\((.*)\))?[ ]*$', re.MULTILINE
+        r'^[ ]{0,3}\[([^\]]*)\]:[ ]*\n?[ ]*([^\s]+)[ ]*(?:\n[ ]*)?((["\'])(.*)\4[ ]*|\((.*)\)[ ]*)?$', re.MULTILINE
     )
 
     def test(self, parent, block):

markdown/extensions/fenced_code.py

@@ -42,13 +42,13 @@ def extendMarkdown(self, md):
 class FencedBlockPreprocessor(Preprocessor):
     FENCED_BLOCK_RE = re.compile(
         dedent(r'''
-            (?P<fence>^(?:~{3,}|`{3,}))[ ]*                      # opening fence
-            ((\{(?P<attrs>[^\}\n]*)\})?|                         # (optional {attrs} or
-            (\.?(?P<lang>[\w#.+-]*))?[ ]*                        # optional (.)lang
-            (hl_lines=(?P<quot>"|')(?P<hl_lines>.*?)(?P=quot))?) # optional hl_lines)
-            [ ]*\n                                               # newline (end of opening fence)
-            (?P<code>.*?)(?<=\n)                                 # the code block
-            (?P=fence)[ ]*$                                      # closing fence
+            (?P<fence>^(?:~{3,}|`{3,}))[ ]*                          # opening fence
+            ((\{(?P<attrs>[^\}\n]*)\})|                              # (optional {attrs} or
+            (\.?(?P<lang>[\w#.+-]*)[ ]*)?                            # optional (.)lang
+            (hl_lines=(?P<quot>"|')(?P<hl_lines>.*?)(?P=quot)[ ]*)?) # optional hl_lines)
+            \n                                                       # newline (end of opening fence)
+            (?P<code>.*?)(?<=\n)                                     # the code block
+            (?P=fence)[ ]*$                                          # closing fence
         '''),
         re.MULTILINE | re.DOTALL | re.VERBOSE
     )