Riot config server name

This adds into the Riot config.json the field
  'default_server_config.m.homeserver.server_name'
with, by default, the value of the playbook's 'matrix_domain' variable.

Riot displays this string in its login page and will now say 'Sign in to
your Matrix account on example.org' (the server name) instead of 'Sign
in ... on matrix.example.org' (the server domain-name).

This string can be configured by setting the playbook variable
  'matrix_riot_web_default_server_name'
to any string, so we can make Riot say for example 'Sign in ... on Our
Server'.

This change makes no difference in itself, but is a pre-requisite for
adding the 'server name' config, and the old syntax is deprecated.

Upgrade riot-web (1.5.9 -> 1.5.10)

Configuring webhooks

Added steps to configure appservice webhooks in the dimension manager.

Fixed appservice webhooks container port mapping

add synapse config enable_group_creation

Update synapse (1.10.0 -> 1.10.1)

Update certbot (1.0.0 -> 1.2.0)

Update nginx (1.17.7 -> 1.17.8)

Update synapse (1.9.1 -> 1.10.0)

matrix-nginx-proxy: adding additional configuration blocks

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tedomum repo contains out-of-date documentation.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