Use `page_action` instead of `browser_action`

I think it would be more consistent to use `page_action` instead of `browser_action` (eg moving the icon in the address bar):
- The wallabag icon would be closer to the bookmark icon
- It's a conceptually similar to bookmarking, so it feels like it belongs in the address bar as well
- Pocket will work this way in future versions of Firefox:


Start by locating the extension manifest and any references to browser_action in the JavaScript extension code. Check how the current address-bar icon is configured, then verify that the icon uses page_action consistently in the supported browser environments.

Written by the indexing model from the issue text.