Refactor authentication

[Edznux]

Follow up of this #286, happy to discuss.

This PR adds:

• exports errors (so we can use errors.Is() in other package)
• Add tests
• Extracts SQL Queries

Notes:
The line token := c.Request.Header.Get("apitoken") is IMO missleading: the actual header is "Apitoken" because it's canonical-ified, and thus is equal to token := c.Request.Header.Get("Apitoken") (note the capital A)
What do you think about updating it to the last one to match what's the user is supposed to provide? (Apitoken and not apitoken)

[its-a-feature]

I'm a little confused about the header update you're mentioning. From what I could see, HTTP header values are case insensitive, so it shouldn't matter? Mythic's Scripting PyPi package for example (https://github.com/MythicMeta/Mythic_Scripting/blob/master/mythic/mythic_utilities.py#L52) just sends the header along as apitoken. Are you suggesting updating it to Apitoken just to match the normal canonical style of capitalization?