Less hardcoded stuff

[dorodere]

Closes #2.

• Whether or not a user is listed on the registration page for a drive is controllable per-user now.
  • As this needs a new column, this PR also adds a new mechanism for database updates, to build upon when versioning and creating update mechanisms.
• The slightly uncomfortable local patch to switch into testing mode is automatically done by detecting Rocket's profile now. By default cargo run turns on the testing mode, cargo run --release turns it off.
  • This is configurable at runtime using the ROCKET_PROFILE environment variable.
  • Testing mode turns off sending emails at the configured domain and instead just logs the login link instead.
• The outside facing address is configurable now.

[dorodere]

Blocking #6 which needs to be done soon-ish. As this is very likely to get merged, I'll base the branch for #6 on this.

[dorodere]

I just realized this is not finished. We can still make this better by adding a settings table, configurable by the enduser, for stuff like the message at the bottom of the page when logging in.

[dorodere]

Actually, no. This will be a lot more work and is mostly unrelated to the other stuff happening here. I'll open an issue with the information here and create a separate PR.