Improvements for the equations plugin

[kulla]

Included in this PR:

• Make explanation field focusable with "tab" key
• Omit sign field with tab
• Add button "add new equation"
• Start with two lines of equations
• Fix bug with missalignment and \underbrace and \overbrace
• Variable length down arrow

TODOS:

• [x] Add "Add new equation" button

[kulla]

@inyono Ready for feedback -> Please do not merge since I want to ask for feedback of the authors too

[kulla]

@inyono I have got positive feedback. After merging https://github.com/serlo/frontend/pull/1262 this PR can also be merged.