Backend pagination POC

[rgmz]

This is a proof-of-concept for supporting backend pagination. It fixes #33.

Most of the heavy lifting is done by Flask SQLAlchemy

TODO (?):

• [ ] Implement pagination for all endpoints

Frontend

• [ ] Create a common component for a paginated table (?)
• [ ] Tweak implementation to closer match this (?)
• [ ] Add loading icon to provide feedback
• [ ] Fix pagination total count + number of pages (currently just says 1-n of n, where n is the number of items for a single page)
• [ ] Surface any backend errors via a popup (or something; don't fail silently)
• [ ] Add debounce for filter input

Backend

• [ ] Update backend responses to include total count of items