Add listing of blobs in management page

[RodionGork]

The problem statement and problem checker are stored as blobs in the table of the same name. They are created and edited with the dedicated page. However we have no feature to list
existing blob keys yet. Let's add this.

Steps

Login as testadmin with the password 1234567. Type in the "very secret" url of management
root page, http://localhost:5000/mgmt - here click the link to blobs page.

Entering t.prg-1.en as a blob key and pressing Load will load the blob for the problem prg-1,
for example.

To add

Now improve this so that if the blob key entered for "Load" operation contains wildcards (stars)
then the list of matching blob keys from database is returned. E.g. entering t.prg-1.* should return
list of t.prg-1.en and t.prg-1.chk.