! Assorted merge-related fixes

This fixes #1792 and adds some minor UI tweaks as well.

Fixes:
1. Clicking "Merge Topics" from within a topic resulted in an error if it was the only topic in that board but you could still select other boards.
2. The board selection form caused a session error when selecting a different board due to nested HTML forms (wrong form was being submitted).

UI tweaks:
- Show a friendly message instead of a page index and empty box with "Merge Topics" button when there aren't any topics in the selected board
- Minor tweaks to the board selection form to account for it looking differently when not nested inside a nested form
- Add standard "button_submit" class to board selection form button
- Add ID to form and tweak index.css to prevent previously-mentioned button from being floated to the right

Signed-off-by: Michael Eshom <oldiesmann@oldiesmann.us>