Only import jinja2 when needed (#5514)

Fixes https://github.com/matrix-org/synapse/issues/5431

`jinja2` was being imported even when it wasn't strictly necessary. This made it required to run Synapse, even if the functionality that required it wasn't enabled. This was causing new Synapse installations to crash on startup.

Email modules are now required.

changelog.d/5514.bugfix

+Fix bug with `jinja2` preventing Synapse from starting. Users who had this problem should now simply need to run `pip install matrix-synapse`.

synapse/python_dependencies.py

@@ -72,10 +72,11 @@ REQUIREMENTS = [
     # Twisted 18.7.0 requires attrs>=17.4.0
     "attrs>=17.4.0",
     "netaddr>=0.7.18",
+    "Jinja2>=2.9",
+    "bleach>=1.4.3",
 ]
 
 CONDITIONAL_REQUIREMENTS = {
-    "email": ["Jinja2>=2.9", "bleach>=1.4.3"],
     "matrix-synapse-ldap3": ["matrix-synapse-ldap3>=0.1"],
     # we use execute_batch, which arrived in psycopg 2.7.
     "postgres": ["psycopg2>=2.7"],