feat(config): add last N limit mapping (jitsi#7422)

Adds 'lastNLimits' config value which allows to define last N value per number of participants.
See config.js for more details.

babel.config.js

@@ -0,0 +1,29 @@
+// babel is used for jest
+// FIXME make jest work with webpack if possible?
+module.exports = {
+    env: {
+        test: {
+            plugins: [
+
+                // Stage 2
+                '@babel/plugin-proposal-export-default-from',
+                '@babel/plugin-proposal-export-namespace-from',
+                '@babel/plugin-proposal-nullish-coalescing-operator',
+                '@babel/plugin-proposal-optional-chaining',
+
+                // Stage 3
+                '@babel/plugin-syntax-dynamic-import',
+                [ '@babel/plugin-proposal-class-properties', { loose: false } ],
+                '@babel/plugin-proposal-json-strings',
+
+                // lib-jitsi-meet
+                '@babel/plugin-transform-flow-strip-types'
+            ],
+            presets: [
+                '@babel/env',
+                '@babel/preset-flow',
+                '@babel/react'
+            ]
+        }
+    }
+};

config.js

@@ -212,6 +212,22 @@ var config = {
     // Default value for the channel "last N" attribute. -1 for unlimited.
     channelLastN: -1,
 
+    // Provides a way to use different "last N" values based on the number of participants in the conference.
+    // The keys in an Object represent number of participants and the values are "last N" to be used when number of
+    // participants gets to or above the number.
+    //
+    // For the given example mapping, "last N" will be set to 20 as long as there are at least 5, but less than
+    // 29 participants in the call and it will be lowered to 15 when the 30th participant joins. The 'channelLastN'
+    // will be used as default until the first threshold is reached.
+    //
+    // lastNLimits: {
+    //     5: 20,
+    //     30: 15,
+    //     50: 10,
+    //     70: 5,
+    //     90: 2
+    // },
+
     // // Options for the recording limit notification.
     // recordingLimit: {
     //

jest.config.js

@@ -0,0 +1,9 @@
+module.exports = {
+    moduleFileExtensions: [
+        'js'
+    ],
+    testMatch: [
+        '<rootDir>/react/**/?(*.)+(test)?(.web).js?(x)'
+    ],
+    verbose: true
+};