Add a selection mode/mechanism to the main Index

[Difegue]

Current UX for quickly working with a lot of archives is painful in a few ways, so this is a design draft to try and fix that.
I'd already briefly mentioned it here: #570 (comment) , but it's more generic than just batch operations and doesn't require a heavy frontend refactor:

• Current Index page should have a new toggle to enable "Multi-Select Mode"
  • (or maybe make it one of the carousel dropdown options? I'm worried about discoverability if doing it that way tho)
• When this toggle is enabled, the topmost carousel is emptied out, and checkboxes appear next to each archive.
• Search works as normal -- Ticking a checkbox adds it to the carousel for quick review of your selected archives, no matter what you're searching for.
• Once at least one file is selected, you can choose to run Batch Operations on the selection.
  • (which will probably be a popup with an iframe to the current batch page or something because that's entirely too much complexity to move into the index)
  • Some simpler options could eventually live here alongside Batch, like quick deletion, creating/adding to a category(or meta-archive when those come!), or IceBreeze's crazy poweruser batch syntax mentioned here