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

• The code change is tested and works locally.
• There is no commented out code in this PR.
• No lint errors (use flake8)
• 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 ✅

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

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Additional details and impacted files

@@            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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