[FEATURE] Bulk-send books via email

[aaronleopold]

Relates to #258

Describe the solution you'd like

I would like the ability to select multiple books at once and send them to an email address

Additional context

As of #296, you will be able to navigate to the book overview page and send it using the configured emailer for the server (assuming you have permissions). The API for sending attachment-based emails was implemented to support multiple, already. So this issue is primarily UI work to:

• Decide on designs for bulk selection of books
• Implement bulk selection of books
• Implement an actionable option during bulk selection for emailing the selection

I'll aim to complete #292 first, and then start roughing out the designs. If anyone has input, please comment!