hub: make footer of the e-mail notifications

[siteshwar]

... configurable through settings.

Resolves: https://issues.redhat.com/browse/OSH-252

[siteshwar]

I have modified this pull request to also include footer for the tasks (non-errata scans).

Example Task (non-errata scan) notification:

---------- MESSAGE FOLLOWS ----------
b'Content-Type: text/plain; charset="utf-8"'
b'MIME-Version: 1.0'
b'Content-Transfer-Encoding: 7bit'
b'Subject: Task [#7] units-2.22-6.fc39 finished, state: CLOSED'
b'From: no-reply@example.org'
b'To: user@example.com'
b'Date: Thu, 04 Apr 2024 16:16:27 -0000'
b'Message-ID: <171224738772.3382.8612301540682693862@osh-hub>'
b'X-Task-ID: 7'
b'X-Task-State: CLOSED'
b'X-Task-Owner: user'
b'X-Scan-Build: units-2.22-6.fc39'
b'X-Application-ID: OpenScanHub'
b'X-Hostname: osh-hub'
b'X-Peer: ::1'
b''
b'Hostname: osh-hub'
b'Task ID: 7'
b'Build: units-2.22-6.fc39'
b'Task state: CLOSED'
b'Task owner: user'
b'Task method: MockBuild'
b''
b'Task URL: http://osh-hub:8000/task/7/'
b'Comment: '
b''
b''
b'All defects'
b''
b'CLANG_WARNING             13'
b'COMPILER_WARNING          4'
b'CPPCHECK_WARNING          5'
b'GCC_ANALYZER_WARNING      6'
b''
b'This is a test footer.'
------------ END MESSAGE ------------

[kdudka]

Looks good. Thanks!

[lzaoral]

@siteshwar Please, always wait for the Lint job to finish before you merge!! https://github.com/openscanhub/openscanhub/actions/runs/8876473051/job/24368050609