Add Admin auth check on /subjects endpoints

[mattrent]

This PR adds an authentication check on /subjects endpoints, using a special admin account. Previous endpoints keep the same authentication mechanism as before.

Specifically, the PR:

• adds the admins table and schema in SubjectsRepo, to contain admin accounts
• adds a token for the admin account when seeding the database, to ensure that at least on admin exists when the application runs
• saves the admin and guest tokens in a local file, readable by the user deploying the platform

Additionally:

• updates Elixir and Alpine versions in Dockerfiles
• removes Rust installation (unused) from Dockerfiles
• fixes SubjectsRepo seeding in Dockerfiles and releases, by starting Plug.Crypto.Application before generating the tokens