Add notification template without escaping

[patrickmann]

Resolves Graylog2/graylog-plugin-enterprise#6525

Provides a notification template that does not escape parameter strings. This is useful if the message is supposed to contain HTML characters, e.g. "Index <i1> is unavailable".

How Has This Been Tested?

Unit test and hand-crafted notifications

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[mpfz0r]

We need to escape the content of the notifications,
as it is untrusted.

I've created an output with an html injection name to demonstrate this:

[patrickmann]

So the requirement is:

• titles are not escaped and not interpreted as HTML
• descriptions are escaped and interpreted as HTML
  We can use Freemarker directives to assign plaintext format to the titles; and leave descriptions as is (HTLM output format).

[mpfz0r]

So the requirement is:

* titles are not escaped and not interpreted as HTML

I wouldn't make the distinction between titles and descriptions.
Titles just shouldn't be escaped twice . I suggest we just treat them as raw html like we do for the description.
See my comment and patch on the other PR.

* descriptions are escaped and interpreted as HTML
  We can use Freemarker directives to assign plaintext format to the titles; and leave descriptions as is (HTLM output format).

[patrickmann]

I went with your initial suggestion of modifying rendering of titles in the FE. It achieves the desired outcome and doesn't require any changes to templates.

[patrickmann]

Looks good, works as expected.