Introduce substitution to do list

Closes #225 and #216

See merge request !329

Resolve "Saving preferences isn't possible due to wrong choice validation"

Closes #275

See merge request !397

Fix substitutions for supervisions

See merge request !396

Show events with current changes in substitutions table

See merge request !394

Resolve "Show only teachers with courses in the current school term"

Closes #274

See merge request !395

Load avatar of affected teachers

See merge request !392

Resolve "Adapt to active school term"

Closes #260

See merge request !380

Merge branch '263-loading-animation-for-timetable-object-select-on-left-side-of-timetables' into 'master'

Resolve "Loading animation for timetable object select on left side of timetables"

Closes #263

See merge request !393