make username comparison case insensitive

[jku]

@SantiagoTorres in signer configuration and @santiagotorres in metadata should be considered the same user

[jku]

There's multiple places where we do this comparison. Two ways to go about it:

• switch all comparisons to not do if s == user.name and instead create a User.matches(s) that does the right thing -- the issue here is that future code must also remember to use this method...
• alternatively make sure user-name is deserialized to lowercase when the config file .tuf-on-ci-sign.ini is read, and that tuf-on-ci-delegate input that asks for new signer names converts to lowercase before storing the signer name. This should ensure we always only process lowercase -- minor downside is that signer names printed on CLI will then always be lowercase

I think the second option is easier to implement so I will go with that.