Add /until command

[TimJentzsch]

Relevant issue: Closes #38.

Description:

Adds an /until command to predict how long it will take to reach a certain gamma score or flair rank.

Note: This PR has been assembled over a longer time frame, with a big merge commit in-between. I think a thorough review would be good to avoid unexpected behavior/bugs. There might also be some line-ending changes (CLRF to LF), I advice turning off the whitespace diff in GitHub (settings cog) if that happens.

Screenshots:

Future Work

The following features will be moved to separate issues / PRs:

• Providing different time frames to calculate the rate on (probably using a similar system as for /history and /rate.
• Providing a user as goal. The bot will then calculate when the user with lower gamma will catch up with the user with higher gamma, taking both their transcribing rates into account.
• Drawing a graph for the predictions. It should include the history graph for the user in the time frame that the rate is calculated on and a dotted line for the predicted transcribing rate until the goal is reached.

Checklist:

• Code Quality
• Pep-8
• Tests (if applicable)
• Success Criteria Met
• Inline Documentation
• Wiki Documentation (if applicable)

[itsthejoker]

plz resolve conflicts