Send notification when status of enrolment changes

Closes #608 (closed)

Design: Step 1: Otto approves student to participate in the project (with no motivation) Screenshot_44

User receives notification: Screenshot_45

Step 2: Otto returns status to pending with motivation: Screenshot_47 User receives such notification: Screenshot_46

Step 3: Otto changes motivation but does not change status: Screenshot_49 User receives notification: Screenshot_48

In the panel: Screenshot_50

Edited by Anna Kalandadze

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