Make base_route_name optional for Config Mappers

Problem/Motivation

Config translation requires that a base_route_name is provided for the config object being translated.
This is used to build up the UI paths for translating the object:
$this->getBaseRoute()->getPath() . '/translate/{langcode}/add',
However, it provides it's own UI for those paths, and doesn't actually use the route specified.

This means that for modules like Views or Page Manager that ship with their UI as an optional module, you cannot translate those entities without their UI modules installed.

Proposed resolution

When a base_route_name isn't provided, use a default path within the /admin/config/regional/config-translation section of the website.

Remaining tasks

Do it!

User interface changes

Config translation will be available even when optional UIs are turned off.

API changes

\Drupal\config_translation\ConfigMapperManager::processDefinition() will no longer throw an exception, which is neither documented nor explicitly tested.

Data model changes

N/A