[FIX] Redefining h[1-6] size
Add support for Jinja2

- templates are searched in indico/web/templates/
- bytestrings are converted to unicode automatically EXCEPT for
  {% trans %} arguments - use |ensure_unicode manually there
- additional functions available globally: url_for (with indico extras),
  url_rule_to_js, formatDateTime, formatDate, formatTime
Use new date/time format functions in Jinja

Also ensure that the result of a custom filters is unicode
Add function to access jinja macros from python

e.g. get_template_module('foo.txt').some_macro()
Separate module for jinja stuff, underline filter
[NEW] Template for alert email agenda
[IMP] Alarm email rendered with Jinja
[REF] Removed unused methods for alarm email
[IMP] Alarm email is now markdown correct
[IMP] Alarm email: address rendering
[IMP] Alarm email: titles with underline filter

Closes #1503
[FIX] Alarm email dealing with empty fields
[IMP] Alarm email agenda rendering

- Markdown compatibility
- All bullets use now the same character
[IMP] Alarm email: placeholder for empty agenda
[REF] Some improvements on the alarm emails

- use implicit falsiness of ''/None/[]
- better wording if there's only an address but no room/location
[REF] Remove unused imports and docstring abuse
[IMP] Recording and Webcast request disabled

- when there are no contributions
Make long blocks more visible in week style
Week TT: Different tooltip style, hover effect
Do not fail hard if plugin blueprint import fails

Uninstalling an enabled plugin should not completely break indico
startup...