Refactoring code / clean up technical debt

[thorbjoernl]

A number of special cases have accumulated which could be cleaned up a bit:

• aerovaldb/src/aerovaldb/jsondb/jsonfiledb.py

  Line 352 in bc68cfb

  if ext.lower() in IMG_FILE_EXTS:

• Using overridden implementations for get/put report_images/mapoverlay (I think these can be simplified to use the regular routing mechanism and reduce duplicate code).

• aerovaldb/src/aerovaldb/sqlitedb/sqlitedb.py

  Line 310 in bc68cfb

  if table_name == "reportimages":