#149 Adjustment of database schema

Database schema is adjusted to ensure consistency of the data.

• Ensure that any branching of dependencies does not lead to inconsistent references
• Revise table and field names in the database
• Revise data constraints on database level (what should be unique, non null, ...)
• Drop measurement.lanes (text field)
• Drop missing relations in the code

Main changes are:

• Many constraints added on database level
• Some tables and fields lost redundant gel_ prefix
• measurement table lost direct foreign key relationship with gel table. This resulted in simplification of gel table UI
• measurement table lost lanes text field which was automatically filled. UI has been adjusted accordingly

Current status: ready for testing and review.

Fixes: #149 (closed) #156 (closed)