Add configuration for pinging committee in welcome messages or not

## Actual Incorrect Behaviour


## Expected Behaviour

* [x] The role name should be correctly linked
* [x] 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