Improve Map Layout Custom Layers Functionality

[swainn]

• Fixed bug with a custom layer being added to the m_map twice.
• Add methods to MapLayout mixin for building custom layers and layer groups consistently
• Remove hard-coded values from add custom layer modal
• Add custom layers to the bottom of the stack of layers (not on top)
• Add layer tree item to the bottom of the Custom Layer group instead of on top
• Make map extent definition easier to change dynamically (at runtime)
• MVView can now be defined from center and zoom OR extent
• Added JavaScript API for adding alert messages dynamically on the front end that are consistent with those generated by Django messages on the backend.
• Map Layout can now automatically discover WMS layer extents for the zoom-to-layer feature.
• Layer groups can be set to be collapsed initially on map layout.
• The rename and remove options can be enabled/disabled on a layer-by-layer basis.
• Legends can be hidden on a layer-by-layer basis.

[coveralls]

Coverage decreased (-0.01%) to 99.99% when pulling a42011b989807f315c5a7db8d9f4a3458adfd7d0 on map_layout_custom_layer_fixes into 6203c3b0181f7d5affcea9bee665173d07472c61 on main.