Refactored the way Notifiarr Discord users are mentioned

[caronc]

Description:

Related issue (if applicable): n/a

The concern came from the Notifiarr Discord chat where one would invoke discord mentions from Apprise.

Checklist

• [x] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@notifiarr-discord-mentions-refactor

# Test out the changes with the following command:
apprise -t "Test Title" -b "Hi <@123>!" \
  notifiarr://webhook/discord_channel_no/

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.26%. Comparing base (6733953) to head (f28615d).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1153 +/- ## ========================================== - Coverage 99.28% 99.26% -0.03% ========================================== Files 142 142 Lines 18440 18428 -12 Branches 3763 3761 -2 ========================================== - Hits 18309 18292 -17 - Misses 121 124 +3 - Partials 10 12 +2 ```

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