Improve restriction for adding same link

[nyudenkov]

Bot already checking for the same link in handler (app/bot/handlers/link.py:45).
Maybe it would be nice to move check in method for Link model class (app.database.models.Link)

Cover next cases:

• Slashes (?) (fox ex. now fmajesty.xyz and fmajesty.xyz/ is not the same link)
• Redirects (for ex. user trying to add redirect.to/fmajesty.xyz link where there is redirect to fmajesty.xyz/ and we have record in DB with link fmajesty.xyz, so we need to restrict adding that link)