More discoverable multi-select and bulk operations

[ChristophWurst]

Problem:

Users didn't know that it is possible to select multiple messages and perform bulk operations such as move, archive, marks as spam or delete.

[marcoambrosini]

@ChristophWurst wasn't this tracked in another issue?

[ChristophWurst]

It's #10194

[jancborchardt]

@marcoambrosini @nimishavijay since neither @ChristophWurst nor me were in the brainstorming session, could you fill the description with more detail on what this is about? :)

[marcoambrosini]

Design call outcome:

• On desktop: show checkbox on hover (either overlay or replace avatar)
• On mobile: click on avatar
• Remove select action from the actions menu
• Take important bulk actions out of the 3 dot menu