Improve UI accessibility - Focus on screen reader support

[mbrzakovic]

Goes one step forward towards a vision shared in #7713.
This change implements several accessibility principles with the focus on improving screen reader support.
Additionally, few UI inconsistencies have been addressed as well.

Changes:

• Ensures elements have discerniable text - Provided missing titles and aria-label to several items (mostly buttons) + missing tooltip added for add tags [fef933a - 094ace6]

• More descriptive titles 10fad14 02e92d9
• Few image alts added a77840e
• Button role added where appropriate 4bee356 50de029 0413320
• Ensure states are being announced - aria-selected/disabled/expanded/pressed etc. e91dc1b 6337e4d
• Polish up the headings (this impacts the speed at which assistive technologies allow people to orient around locations on page) 30ef0b8 8eba5db. Additionally, keyboard shortcuts heading has been centered, and sidebar headings have been increased, making the ui better looking and more intuitive.

[tyrasd]

Thank you for the pull request. These are certainly good improvements. I've got only one question/comment, see below.

PS: I've fixed the merge conflicts caused by other changes in the development branch. Also, I've added a line to the changelog to this branch (see 960f576). Feel free to amend it if you think something should be mentioned more specifically.