Add create Poll to Threads

We removed the ability to create a poll from the first version of Threads for simplicity. It is our intention to enable this feature in the future.

https://github.com/vector-im/element-meta/issues/144
https://github.com/vector-im/element-meta/issues/162

## Original issue below

It's possible to start a poll and use "answer in thread" to kinda pin this poll alongside a discussion in a thread. But the thread view only shows "user created a poll".

If you create a message like "Let's do a poll" -> answer in thread -> create a poll inside thread, it is not possible to do so as the "+" menu has no entry for polls.

This leads to two ways Element should deal with polls in threads:

1. Show poll title in thread list.
or
2. Create a message like "Let's do a poll about blabla" -> reply in thread -> add poll