Activities table row count and inserting

[redcatbear]

Situation

Most current application with tables allow adding new entries by clicking on a free row. I intuitively tried to do that, but it did not work.

Acceptance criteria

1. Limit the number of rows to "current + 1".
2. Write "add new entry" in the first cell of the empty line.
3. Create a new entry when users try to enter data in the empty last line.

[kaklakariada]

In #164 we added a label describing how to add activities. Does this solve this issue?