Adding is_poll to the discussion table

[nodeloc]

Adding is_poll to the discussion table significantly improves the efficiency of polls and avoids redundant queries.

Confirmed

• [√ ] Frontend changes: tested on a local Flarum installation.
• [√ ] Backend changes: tests are green (run composer test).

[dsevillamartin]

This only handles the initial discussion creation with a post. If I remember correctly, polls can now be deleted as well.

Also not sure I love this, but I also can't think of another way to achieve it, since the poll data is present on the first post relationship anyways.

Perhaps the column should also be has_polls to be more precise, if we choose to go with it.