WIP: Chats typing indicator

Fixes #488

Developed proof-of-concept typing indicator for chats.

Uses a GIF to indicate remote typing activity. Uses the chats API to deliver a special chat content that is rendered as a typing activity indicator.

This will only work properly between Soapbox FEs that run this code. To be successful, it will require typing indicator signaling to be added to Pleroma BE.

This can be used as a demonstration of the viability of the concept, then cleaned up once the BE support becomes available.

Renders typing activity for 5 seconds after receipt of signal. Outputs typing activity signal no more frequently than once per 5 seconds, while typing a message.