Long translatable string in user interface translations

Problem/Motivation

Upon installing this module, the config item "notifications_widget.settings:excluded_entities" adds a long unbroken translatable string to the user interface translation.

There are two problems with this:

1. The user interface translation table gets stretched out, the left column being incredibly long leaving nearly no space for the content input on both Claro and Gin
2. Config should not be translatable in the user interface translation, the translation is luckily not used anywhere, but it still shouldn't be in there.

Steps to reproduce

1. Install the module
2. Visit the user interface translation page
3. If the string isn't on the first page, look for the default value by searching for: "block,block_content_type"

Proposed resolution

We should change the data type from "text" (which is translatable) to "string" (which isn't).

Data model changes

The "excluded_entities"-key in the "notifications_widget.settings"-config should be a non-translatable data type.