Not sure if we need to keep the conditional requirement name though...

Create a generic function to read template files, which are also treated as
jinja2 templates whether they have any custom variables or not. The goal is
for this function to be used across the codebase. At the moment it is
located in emailconfig.py, but I'd like to move it to another config file
at some point along with the rest of the template options.

We'll be using it in other files in subsequent commits

jinja2 and bleach are no longer optional dependencies.

Also remove guard on importing load_jinja2_templates here

…ault_templates

* 'develop' of github.com:matrix-org/synapse: (57 commits)
  Remove a space at the start of a changelog entry.
  More changelog tweaks
  More changelog tweaks
  Remove unwanted changelog line
  1.19.0rc1
  Run `remove_push_actions_from_staging` in foreground (#8081)
  Drop federation transmission queues during a significant remote outage. (#7864)
  Fix unawaited coroutine error in tests. (#8072)
  Convert the roommember database to async/await. (#8070)
  Convert devices database to async/await. (#8069)
  Add type hints to handlers.message and events.builder (#8067)
  Convert account data, device inbox, and censor events databases to async/await (#8063)
  Convert appservice, group server, profile and more databases to async (#8066)
  Fix typing for notifier (#8064)
  Convert tags and metrics databases to async/await (#8062)
  Converts event_federation and registration databases to async/await (#8061)
  Add comment explaining cast
  Update changelog.d/8051.misc
  Add typing info to Notifier (#8058)
  Handle optional dependencies for Oidc and Saml
  ...

Turns out jinja2.FilesystemLoader can take a list of directories to look for
a given filename in, so there's no need to check for the file ourselves.

We also now reuse the jinja2.Environment object

The urllib -> urllib.parse switch was because we use urllib.parse in the code
which afaik only works because we're importing urlib.parse elsewhere, which
is flakey

So that people are warned early that their assumed custom
template directory is in fact doing nothing

Lots of general cleanup of the template code in emailconfig.

The account_validity config option bit was that I noticed what we were
pulling out of the config had been done elsewhere already.

Also move filter functions from synapse.push.mailer to synapse.config._base
as otherwise we were facing circular imports (plus it makes sense to keep them by
read_templates anyhow).

The performance difference is negligible and cleans up some duplicate code.

Otherwise a TypeError will be raised if template_dir is uncommented
but not set to anything

Additionally remove default_template_dir here as read_templates
handles that for us.

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

We switched from reading them directly from disk to making them
into jinja2.Template files, which broke stuff. We now render
them to turn them back into text.