[cms] ui/ux improvements: create invoice

[MariaPleshkova]

Suggesting a set of improvements for "Create Invoice" page.
Figma: https://www.figma.com/file/RuzhkKo9fUjKQLqGbUPjHl/decred---piui---cms?node-id=707%3A2406

• page has been split into 2 sections: Invoice Details and Invoice Summary
• Invoice Details section has 3 parts now:

The 1st one includes Reference Month dropdown and an Exchange Rate,

The 2nd one includes input fields with contractor details and an area to attach files

By clicking on "Use contractor details from the previous invoice" button you can paste previously entered info. Also there is a button for pasting a copied payment address within the "Payment Address" field.

The area with attaching a file can fit 6 file previews without changing it height:

If you add more than 6 then this area will stretch down:

The 3rd part includes a button to add proposal items and some info that was within a table before and is basically a replacement of a previous table view of it.
The pros of using this new view are mainly:
-Amount of clicking effort adding new items (reducing repetition, etc)
-Card components being properly responsive, opposed to tables
-Overall clearer visual grouping between the entry values; not having the need to display what is not necessary

You can choose a domain within a tab and a proposal. "Choose proposal" field is an input field with filtering option. When you click on it, it shows a dropdown with proposals within the chosen domain. Maybe they can be sort by popular ones (mostly used by you, for example) or just from a to z. When you start typing something then only relevant results will be displayed there.

When you have chosen a domain and a proposal and then clicked on Add Item button new cards will appear under this section.

The Domain and Proposal fields remember the first selection from the previous step. They have not to be resetted after clicking on the "add item". This way we can reduce the amount of clicks here, because there will be no need to repeat this info again.

Also, please, note new fields used here. We use similar line fields like before, but without a title above. And the Domain tab has smaller height than tabs we used before. So basically they are kind of smaller versions of field/tab elements we had before.
These would be additional elements and they shouldn't replace the other similar fields/tabs used in the piui.
(all of these can be found within the Components page in the Figma file)

• Invoice Summary section shows all totals for the current invoice:
  

When you have already billed an item/items to proposal in the current invoice, proposals names and the number of items billed to them will be displayed next to "Items billed to proposal":