fix slack bridge: salt too large, sha512_crypt requires <= 16 chars

Fix matrix-appservice-irc database specification

This fixes an incorrect indentation in the database specification for
appservice-irc which caused matrix-appservice-irc to refuse to start
with the remarkably unhelpful error message:

```
ERROR:CLI Failed to run bridge.
```

This also updates doc links to the new matrixdotorg repo because the
tedomum repo contains out-of-date documentation.

fix invalid password salt on slack bridge

Regression since #362 (Github Pull Request).

Switch from kamax-matrix/matrix-synapse-rest-password-provider to ma1uta/matrix-synapse-rest-password-provider

Switch to upgraded & official matrix-appservice-irc Docker image

Link to official instructions on how to install Ansible

This reverts commit 61e93744ec.

Reverting because of:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341
- https://github.com/matrix-org/matrix-appservice-slack/issues/347

Synapse v1.9.0 changed some things which made the REST Auth Password
Provider break.

The ma1uta/matrix-synapse-rest-password-provider implements some
workarounds for now and will likely deliver a proper fix in the future.

Not much has changed between the 2 projects, so this should be a
painless transition.

This change allows us to work with both our existing Docker image
(`tedomum/matrix-appservice-irc:latest`) and with the
official Docker image (`matrixdotorg/matrix-appservice-irc`).

The actual change to the official Docker image requires more testing
and will be done separately.

Fix for when if case evaluates to false

Install python3-docker if ansible is running python 3

Fix invalid password salts for slack webhooks

Related to #355.

Upgrade Synapse to 1.9.0

Matrix change user admin status

uri.follow_redirects is now a string field

Use Postgres 12.1 when creating a backup

Reword updating-users-passwords.md option 3

Fix regex in matrix_appservice_webhooks_registration_yaml

Related to #339 (Github Pull Request).