This change allows to read any config value from a file, when the path to that file is specified in the config with the target config value suffixed with _file. This works with environment variables as well.
For example, setting database.password_file=/path/to/password will load the value from /path/to/password and set it as the config value of database.password.
Resolves https://kolaente.dev/vikunja/vikunja/issues/704
Resolves https://kolaente.dev/vikunja/vikunja/pulls/1621