Resolve "Indicate job offer sets 'Head TA' role instead of 'TA'"

Closes #192 (closed) Based on !357 (closed) What it does:

• Teacher can create HeadTA-only/TA-only/Either roles students can apply to.
• If teacher changes type of job offer, nothing happens, only when new people accept the job they will be given that new role.
• Promote/demote button only showed for "Either" type of job offers.
• Set defaults to "Either" type of job offers (since this is how it has been done before)
• Testing