[BB-1674] Initial version of pluggable templates

Description

Implements a system that allows plugins to inject static content into all pages.

*JIRA tickets*:

*Screenshots*: Always include screenshots if there is any change to the UI.

*Sandbox URL*: TBD - sandbox is being provisioned.

*Testing instructions*:
TBD

*Reviewers*

  • [ ] @giovannicimolin

  • [ ] edX reviewer[s] TBD

*Settings*
```yaml
EDXAPP_FEATURES:
ENABLE_COMBINED_LOGIN_REGISTRATION: true

EDXAPP_EXTRA_REQUIREMENTS:

EDXAPP_ENV_EXTRA:
ADDL_INSTALLED_APPS:

  • test_template_plugin
    ```

Assignee

Unassigned

Reporter

Open Source Pull Request Bot

Labels

Contributor Name

Kshitij Sobti

Repo

edx/edx-platform

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Priority

Unset
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