This diff introduces `parseUserMentions`. `parseUserMentions` takes the username found in the markdown entity/capture and uses that username to find the id of that user using `memberMap`
This is step 2 in the list below
Outlined below are the steps I will take to make @metion of a user into a touchable
1. Introduce `createMemberMapForUserMentions` function (Right now we only store a set of usernames but we also will want to know the id of the user based on the username)
2. Introduce `parseUserMentions` function (We will use this function to get and return a userID from the `membersMap`)
3. Introduce `userProfileBottomSheetActive` state to the markdown context (This will be used for disabling the press to focus on a message functionality)
4. Introduce `MarkdownUserMention` component (this will render the touchable that will navigate to the user profile)
Depends on D9391