* Add matrix-registration-bot

This adds an install and uninstall task plus helpers. The bot is disabled by default.
This commit does not include documentation, yet. In short, the bot can be enabled by adding
matrix_bot_matrix_registration_bot_enabled: true
matrix_bot_matrix_registration_bot_matrix_user_password: "verysecret"
matrix_bot_matrix_registration_bot_matrix_admin_token: "supersecret"
to the host_vars

* Change bot username to bot.matrix-registration-bot following convention

* Address smaller remarks, fix local docker build

* Switch to an env file

* Add environment variables extension for additional config

* Add documentation for the matrix-registration-bot

* Add screenshot on how to obtain admin access token

* Use bot as admin to only have one access token (bot and admin api)

* Use cleaner setting of matrix_synapse_registration_requires_token

* Use config file for cleaner more secure usage

* Delete unneeded env

* Rename v...