Cleanup the Web interface classes

Description

This MR brings some more cleanup in the Web interface classes after the merge of !129 (merged). Namely,

• Remove unused functions and tabs;
• Use nested namespaces in the Web interface classes;
• Simplify and optimize the tab creator helper class.

In addition, the supervised application state update has been fixed:

• Fix JSON parameter name in GEM monitor update.

Related Issue

How Has This Been Tested?

The Web interface is still functional. State transitions are possible.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.