More user friendly severity level messaging
Included the changes I mentioned in [my earlier comment](https://github.com/alexey-zakharenkov/subways/pull/14#issuecomment-948391863) to:
- Add section headers to the different severity level messages
- Change the errors red colour to a accessible one
- Changed copy of entrance errors from "e:" to "ent:"
- Shortened an error message slightly to reduce possibility of it wrapping into a new line and have the "Under construction" keyword at the beginning of the sentence
CSS property width=fit-content is not supported in Firefox; shorten tooltip text; exclude empty validation message during HTML genertion
Tweaks to the HTML output template:
- Sticky footer so update timestamp always visible without scrolling
- Slight copy update to clarify that networks are shown behind the map link
- table row hover state now uses hue-rotate to ensure enough contrast as compared to whole line blue previously
