Reimplement so that only tab-completed mentions are stripped

Instead of blindly stripping all MD mentions, only strip those that were tab-completed. We do this by adding the `isCompleted` flag to the Entity data.

src/components/views/elements/Pill.js

@@ -92,7 +92,6 @@ export default React.createClass({
                 avatar = <RoomAvatar room={room} width={16} height={16}/>;
             }
         }
-
         const classes = classNames({
             "mx_UserPill": isUserPill,
             "mx_RoomPill": isRoomPill,

src/components/views/rooms/MessageComposerInput.js

@@ -731,10 +731,33 @@ export default class MessageComposerInput extends React.Component {
             sendTextFn = this.client.sendEmoteMessage;
         }
 
-        // Strip MD user mentions to preserve plaintext mention behaviour
+        // Strip MD user (tab-completed) mentions to preserve plaintext mention behaviour
         contentText = contentText.replace(REGEX_MATRIXTO_MARKDOWN_GLOBAL,
-        (markdownLink, text, resource, prefix) => {
+        (markdownLink, text, resource, prefix, offset) => {
-            return prefix === '@' ? text : markdownLink;
+            // Calculate the offset relative to the current block that the offset is in
+            let sum = 0;
+            const blocks = contentState.getBlocksAsArray();
+            let block;
+            for (let i = 0; i < blocks.length; i++) {
+                block = blocks[i];
+                sum += block.getLength();
+                if (sum > offset) {
+                    sum -= block.getLength();
+                    break;
+                }
+            }
+            offset -= sum;
+
+            const entityKey = block.getEntityAt(offset);
+            const entity = entityKey ? Entity.get(entityKey) : null;
+            if (entity && entity.getData().isCompletion && prefix === '@') {
+                // This is a completed mention, so do not insert MD link, just text
+                return text;
+            } else {
+                // This is either a MD link that was typed into the composer or another
+                // type of pill (e.g. room pill)
+                return markdownLink;
+            }
         });
 
         let sendMessagePromise;
@@ -900,7 +923,10 @@ export default class MessageComposerInput extends React.Component {
         let entityKey;
         let mdCompletion;
         if (href) {
-            entityKey = Entity.create('LINK', 'IMMUTABLE', {url: href});
+            entityKey = Entity.create('LINK', 'IMMUTABLE', {
+                url: href,
+                isCompletion: true,
+            });
             if (!this.state.isRichtextEnabled) {
                 mdCompletion = `[${completion}](${href})`;
             }