Feat: Fix that Emails render all symbols right

[Gepardgame]

Description

Fixes that emails can render all symbols right:

Addressed Issue

Fixes #4140

Additional Details

Changed MIME Type to include chaset=utf-8, so it uses utf-8 and not just ASCII.
Unescapse the content, so e.g. & does not convert to & and other such things.

Checklist

• I have read and understand the contributing guidelines
• This PR fixes a defect, and I have provided tests to verify that the fix is effective
  - [ ] This PR implements an enhancement, and I have provided tests to verify that it works as intended
  - [ ] This PR introduces changes to the database model, and I have added corresponding update logic
  - [ ] This PR introduces new or alters existing behavior, and I have updated the documentation accordingly

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00% (target: -1.00%)	✅ 100.00% (target: 70.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (237ba91)	21956	17319	78.88%
Head commit (014ec6c)	21957 (+1)	17320 (+1)	78.88% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4141)	3	3	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[nscuro]

Looks OK to me. Could you add a test to SendMailPublisherTest that includes special characters, to verify the escaping works as expected?

[Gepardgame]

I don't know why the test fails, locally it runs completely fine.

[nscuro]

Maybe flaky, haven't seen that test fail before but could always be. Restarted the test workflow.