This commit is a part of the efforts to rename btn-related classes
and variables to align with Zulip's no-abbreviations policy and
streamline the search results.
* `.container` class is only used in `/activity/support` and
`/activity/remote/support` pages. It is also used in emails but
it has its own CSS.
* Since the CSS applied is localized and looks good, we just
move the CSS relevant to us to `activity.css` which is used
by both of these pages.
* Searched `"container"`, ` container `, `"container ` and
` container"` to look for the uses of this class.