Severity column icons on admin/reports/dblog are not accessible

Problem/Motivation

The icons the the log table rows at admin/reports/dblog are not accessible.

They are created with CSS background images, and the table cells are completely empty.

Proposed resolution

1. Display the severity as visible text, and treat the icon as a non-essential decoration. Not all severity levels have an icon currently. That's OK, the text will the main version of the content, and this will be available to all users.
2. Add a column header called "Severity" (the same label as the exposed filter, for consistency)

Remaining tasks

• Change the existing empty column in the watchdog view configuration
  • Use a column header called "severity"
  • Put the severity text inside the column as visible text.
• Update CSS from dblog module as necessary.
  • In dblog module
  • In stable theme
  • In stable9 theme
• An update path for the view configuration.
• Write tests

User interface changes

Adds visible text for the log entry severity, in a column of it's own.

API changes

None.

Data model changes

None.

Release notes snippet