Drop Bower support & compatibility mode in Flow 3.0+

[Artur-]

Bower is included in Flow 2 for backwards compatibility with Flow 1. It should be removed from Flow 3.

• Remove "compatibility mode"
• Remove Bower build from the Maven plugin
• Remove webjar support
  • Need to fix DnD mobile polyfills to be included in another way than now
• Remove HTML import support
  • Remove theme translations for html imports
• Remove frontend://
• Remove features related to the Bower bundle, such as BundleDependencyFilter
• Remove any Bower & webjar related documentation
• Remove all OSGi related things (code & documentation)

Excluded:

• Full updating of the migration documentation is not included in this ticket, only mentions to bower&webjars should be removed.
• No need to add any documentation about updating to 15 as part of this ticket

[pleku]

The point

Remove webjars support

would mean that our mobile dnd polyfill should be shipped as @NpmPackage & @JsModule.
Currently it is imported as npm-based webjar and added dynamically in DnDUtil.

[Artur-]

Some old prototyping has been done here https://github.com/vaadin/flow/tree/proto-remove-bower

[Legioth]

If I remember correctly, the mobile dnd polyfill is loaded as a "standard" webjar rather than through our custom webjar support. It is thus possible that it would keep working even if we'd remove our custom logic.