Display user names the same everywhere

[violine1101]

Purpose

Display new user handles as @violine1101 and old ones as violine1101#8993 everywhere.

This also should fix the issue where messages could not be deleted if you had a new user handle.

Approach

Add a helper function in DiscordUtil to decide how to print the name. Also added two more helper function for consistent setting of embed footers and authors.

Future work

We'll probably need to rework this with the next discord.js update anyway, I presume (though I haven't checked their progress on the matter)

[chandler05]

I can take a look at this later, but from what I can tell this would still leave #201 as an issue. Not sure if you'd want to figure out a solution for that here, or if you'd rather leave that alone right now.

[violine1101]

I've specifically tested that case and it should work (though only with my second commit)

[mdashlw]

Missed this:

mojira-discord-bot/src/util/RequestsUtil.ts

Line 75 in 437f7b5

.replace( '{{author}}', `@${ message.author.tag }` )

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discord.js support for pomelo was added in 14.12.0 (currently using 14.2.0). user.tag now returns username or username#tag whereas this getUserHandle returns @username