Bridge join from Gitter to Matrix

Bridge join from Gitter to Matrix so we have a cutover point we can backfill from.

A leave on Gitter is already bridged to Matrix: !2200 (merged) (#2763 (closed))

Part of #2857 (closed)

Dev notes

Data change events that we see when someone on Gitter joins a room:

{
  url: '/rooms/60b180ad29600113e4070311/users',
  operation: 'create',
  model: {
    id: '5f762e89986e461e663059c2',
    username: 'MadLittleMods',
    // ...
  },
  type: 'user'
}

{
  url: '/user/5f762e89986e461e663059c2/rooms',
  operation: 'create',
  model: {
    id: '60b180ad29600113e4070311',
    uri: 'MadLittleMods/feawewfafeffewafewewfaafewaewf',
    // ...
  },
  type: 'room'
}

Unshortened versions

onDataChange user {
  url: '/rooms/60b180ad29600113e4070311/users',
  operation: 'create',
  model: {
    id: '5f762e89986e461e663059c2',
    status: undefined,
    username: 'MadLittleMods',
    displayName: 'Eric Eastwood',
    url: '/MadLittleMods',
    avatarUrl: 'http://localhost:5000/api/private/avatars/gh/uv/4/MadLittleMods',
    avatarUrlSmall: 'https://avatars0.githubusercontent.com/u/558581?v=4&s=60',
    avatarUrlMedium: 'https://avatars0.githubusercontent.com/u/558581?v=4&s=128',
    scopes: undefined,
    online: undefined,
    staff: undefined,
    role: undefined,
    providers: undefined,
    removed: undefined,
    v: 71,
    gv: '4'
  },
  type: 'user'
}

onDataChange room {
  url: '/user/5f762e89986e461e663059c2/rooms',
  operation: 'create',
  model: {
    id: '60b180ad29600113e4070311',
    name: 'MadLittleMods/feawewfafeffewafewewfaafewaewf',
    topic: '',
    description: undefined,
    avatarUrl: 'http://localhost:5000/api/private/avatars/group/iv/4/5f762f7a986e461e663059c7',
    uri: 'MadLittleMods/feawewfafeffewafewewfaafewaewf',
    oneToOne: false,
    userCount: 0,
    user: undefined,
    unreadItems: 0,
    mentions: 0,
    lastAccessTime: '2023-01-17T02:04:57.459Z',
    favourite: undefined,
    lurk: false,
    activity: undefined,
    url: '/MadLittleMods/feawewfafeffewafewewfaafewaewf',
    githubType: 'REPO_CHANNEL',
    associatedRepo: undefined,
    security: 'PUBLIC',
    premium: undefined,
    noindex: false,
    tags: undefined,
    providers: undefined,
    permissions: undefined,
    roomMember: true,
    groupId: 5f762f7a986e461e663059c7,
    group: undefined,
    backend: undefined,
    public: true,
    exists: undefined,
    matrixRoomLink: 'https://matrix.to/#/#MadLittleMods_feawewfafeffewafewewfaafewaewf:my.matrix.host?utm_source=gitter',
    v: undefined
  },
  type: 'room'
}

Data change events that we see when someone on Gitter leaves a room:

{
  url: '/rooms/60b180ad29600113e4070311/users',
  operation: 'remove',
  model: { id: 5f762e89986e461e663059c2 },
  type: 'user'
}

{
  url: '/user/5f762e89986e461e663059c2/rooms',
  operation: 'remove',
  model: { id: 60b180ad29600113e4070311 },
  type: 'room'
}