Add assembly warnings

[sbliven]

Add warnings for assemblies, analogous to the interface warnings.

Example warnings could be:

• Assembly forms a co-crystal
• Assembly forms an infinite fibril
• Some interfaces are not consistent (i.e. invalid assembly)
• This assembly does not cover the whole unit cell (some pdb assemblies)

This would require a new database table, new methods to populate the table after assembly determination, an additional column on the structure page, and an icon on the assembly page (as shown in the mockup).