Render sticker events in the timeline

[lampholder]

This work is scoped to rendering only - we need all Riot clients to render sticker events properly in the timeline before we launch 😄

We can follow up adding the ability to send stickers to the mobile clients later.

Key features

• Render the sticker in the timeline

Nice to have

• When the sticker is selected, also show the sticker description in a pop up bubble that appears above/below the sticker (depending on where the sticker is on the screen). We would really like this feature, but not if it means we can't deliver the key feature within the March 30 sprint.

[lampholder]

Selected view; no description:

[lampholder]

Selected view; with description (above and below):

Key details:

• description appears above or below sticker (in order to be visible)
• description does not need to change its position with scroll
• description sits on top of a full width 100% opaque background-coloured rectangle (to avoid an avatar floating next to the description and it looking like that person said the image description)

[lampholder]

Outstanding items:

• link to the event spec
• confirm with Amandine/Matthew that they're happy with the above for iOS
• confirm with Amandine/Matthew that they're happy with the above for Android
  • this is more controversial, since Android doesn't have the grey speech bubble overlay as part of its UX already

[rxl881]

Thanks Tom. For the record, the PR with the sticker message info. is - matrix-org/matrix-spec-proposals#1158

[manuroe]

There are 2 stickers events after this event: https://matrix.to/#/!tAMgPDeOawUJMkFXVZ:matrix.org/$1521649426549165RCzAn:matrix.org

[heitorPB]

PR matrix-org/matrix-spec-proposals#1158 was merged into master.
Matrix now has some kind of sticker specification.
It is vague enough to support animated stickers, i think. But it says nothing about sticker packs.