[BUG] Users can not upload ssh keys with new crypto e.g. ecdsa-sha2-nistp256

[mdujava]

Describe the bug
Users can not set their ssh key if theirs key type is longer than 16 chars.

Version-Release number
28.*

To Reproduce
Steps to reproduce the behavior:

1. User has ecdsa-sha2-nistp256 key or generates new key with ssh-keygen -t ecdsa ...
2. Go to User preferences
3. Try to add ssh key ecdsa-sha2-nistp256 c3NoIGtleSBkYXRhLCBwIGFuZCBxCg== comment

Actual behavior
500 INTERNAL SERVER ERROR is shown bellow +Add button

Expected behavior
Key is stored.

Additional context
Definition of the key type column.

beaker/Server/bkr/server/model/identity.py

Line 990 in 6fc33b0

keytype = Column(Unicode(16), nullable=False)