Store component state for editors

[lukebarnard1]

to prevent a forceUpdate from /sync causing the editors to revert
before the user had a chance to hit "Save".

Part of fixing element-hq/element-web#6019

[dbkr]

Sorry, I'm failing to understand what the cause of the bug was or how this fixes it. It looks like this is sort of making the component controlled, except not because EditableText doesn't support that and it's instead updating the initialValue prop on each keypress but I don't understand how this helps.

[lukebarnard1]

By storing the values as state on the react component, we persist the changes that have happened so far and by controlling EditableText, we prevent it from reverting to initialValue.

Do you think we should re-implement EditableText to be controlled, storing it's own react state?

[dbkr]

After brief clarification in #riot-dev:matrix.org, it still reverts to initialvalue, but that's now OK because initialValue has been updated whatever the user typed in.

So yeah, this looks fine.