Adds support for marking and handling intense training with a teacher…

[xdy]

… on the character sheet.

Fixes #40

[pafvel]

The pull request is a good POC for the feature but needs som additional work:

• I would prefer to hide this feature behind a feature toggle in settings so that only users that really want it and know what they are doing will enable it. Overall I'm concerned that adding this to the skills tab will confuse users. Improving skills and selecting trained skills has been subject to confusion before, but it has been resolved in recent improvements. I'm worried that adding more things in that area will bring confusion back.
• It doesn't work according to the rules. It should only prevent using a trainer when the skill value increased using a trainer.
• It doesn't use the correct size checkbox and the checkboxes don't align with the icons.
• The trainer checkbox should be a button that is enabled/disabled.

If you want to leave the pull request as is then that's fine - I can use it as a starting point for later.

[xdy]

Makes sense. I'll take a look over the next couple days if you don't get to it first.