Updated edx-proctoring requirement to 0.12.15

Make CMS 500 template safe by default

Make CMS 404 template safe by default

Sstudent/safe video template

making the welcome_back template safe

Merge release to master

Extract hint from html instead of text

Moved white label sites to logistration.

ECOM-2948

Fix styling of vote button in forums responses.

Safe templates for static_templates

Merge private/release into release

Secure templates used to inject Segment and Optimizely

Mark register-sidebar template safe by default

Escape properly the elements on the dashboard xseries upsell template

Safe template linter should use DOTALL

MULTILINE has to do with how '^' and '$' behave, DOTALL will make the
'.' match newlines as well. This catches several failures that were
previously missed.

Fix accidental extra escaping

Make CMS activation_invalid template safe by default

Make CMS activation_complete template safe by default

Add h filter page directive to cms mako templates without variables

add filter to profile page

Minor fixes to the safe template linter.

[TNL-4073][TNL-4273] Make sure that domain defined for preview exists in

Implement 'from_string_or_404' in utils

Remove old teams example templates.

The files to change were found with:
`ack --literal --type=html --match '${' --files-without-matches cms/templates`