Add configuration for pinging committee in welcome messages or not

[CarrotManMatt]

Actual Incorrect Behaviour

Expected Behaviour

• The role name should be correctly linked
• If the role name cannot be linked, the role name should be output, preceded the @ (at) symbol. It should not include the angled braces like it currently does
• An error message is output if TeX-Bot fails to substitute the role name correctly.
• Add config flag for whether the welcome messages should ping committee or not

[Thatsmusic99]

I do hate to be a downer on this, but I felt like this was intentional? This is just my personal opinion but I'm not incredibly fond of being pinged every time this message rolls around - it will probably be funny at first, but I feel like I'd slowly get tired of it.

[CarrotManMatt]

Ah very good point. I probably should have spoken to committee to get their thoughts on whether they wanted to be pinged.

I've just had a proper look and it's actually because of an issue in the original messages.json file, the substitution function code is actually fine. I will make it temporarily never ping committee (just have better formatting) and instead later add a config flag to allow for enabling/disabling it.

[CarrotManMatt]

This now depends upon #165