Renamed some entities to more appropriate names

[stefanoborini]

The following changes have been introduced:

• The term Accounting now refers exclusively to the relationship table connecting users, applications and policies.
• The config file accounting_class and accounting_kwargs are now database_class and database_kwargs.
• AppAccounting is now ORMDatabase. Similarly, CSVAccounting is now CSVDatabase.

The above two break compatibility with the old configuration files. As a result, it's important to bump the version to 2.0

[codecov-io]

Codecov Report

Merging #419 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   95.24%   95.26%   +0.01%     
==========================================
  Files          96       96              
  Lines        4102     4114      +12     
  Branches      255      254       -1     
==========================================
+ Hits         3907     3919      +12     
  Misses        143      143              
  Partials       52       52

Impacted Files	Coverage Δ
...teappmanager/webapi/admin/tests/test_accounting.py	100% <ø> (ø)	⬆️
remoteappmanager/webapi/admin/tests/test_user.py	100% <ø> (ø)	⬆️
...eappmanager/webapi/admin/tests/test_application.py	100% <ø> (ø)	⬆️
remoteappmanager/db/csv_db.py	98.75% <100%> (+0.03%)	⬆️
remoteappmanager/base_application.py	97.67% <100%> (ø)	⬆️
remoteappmanager/db/tests/test_interfaces.py	100% <100%> (ø)	⬆️
remoteappmanager/db/tests/abc_test_interfaces.py	87.01% <100%> (ø)	⬆️
remoteappmanager/file_config.py	96.66% <100%> (ø)	⬆️
remoteappmanager/db/interfaces.py	100% <100%> (ø)	⬆️
remoteappmanager/tests/test_application.py	100% <100%> (ø)	⬆️
... and 10 more

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