Some Member field + history tracking improvements

Proposed changes

Added history tracking to the Member and Machine models (8e85f5646e45639a938c15141a45baae9555b7c7), as well as the following many-to-many fields:
- ContentBox.extra_change_permissions, Question.categories and InheritanceGroup.own_permissions (79bd0d7d2bc2474f391a7e0a1270a5eff8d19891)
- Note that the many-to-many selection for these fields are correctly saved, they're just currently not correctly displayed in the objects' history page in Django admin (the current M2M selection is always shown); see this issue

Added a "help text" yellow question mark icon next to the "Discord username" field in member info forms (displayed when clicked / hovered over) (5f2b7d9b2a4e3a526cf6afb360b9b3db00100d43)

Renamed members' gmail field to google_email, as the email address of a Google user can potentially belong to any host, not just "gmail.com" (da2859f63d0169a1967afc19be1ac3dc17a680cf)

Mostly that the history tracking works as expected.

Should run manage.py populate_history internal.member make_queue.machine (after migrating).

(If any of the points are not relevant, mark them as checked)

[x] Created tests that fail without the changes, if relevant/possible
[x] Made sure that your code conforms to the code style guides and that any common code smells have been addressed
- (It's not intended that you read through this whole document, but that you get yourself an overview over its contents, and that you keep it in mind while taking a second look at your code before opening a pull request)
[x] Added sufficient documentation - e.g. as docstrings or in the README, if suitable
[x] Added your changes to the "Unreleased" section of the changelog - mainly the changes that are of particular interest to users and/or developers, if any
[x] Added a "Deployment notes" section above, if anything out of the ordinary should be done when deploying these changes to the server