pkg/linksharing/sharing: update ui

Update on styles, colors, icons, graphics, layout, responsiveness, etc. Replaced placeholder image (no preview available) with css only.

Code Review Checklist (to be filled out by reviewer)

[ ] Are there any authservice's database migrations? Are they forwards and backwards compatible?
[ ] Does the PR describe what changes are being made?
[ ] Does the PR describe why the changes are being made?
[ ] Does the code follow our style guide?
[ ] Does the code follow our testing guide?
[ ] Is the PR appropriately sized? (If it could be broken into smaller PRs, it should be)
[ ] Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
[ ] Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
[ ] Does any documentation needs updating?
[ ] Do the database access patterns make sense?
[ ] Copy the Commit Message Body section contents into the submit prompt upon PR completion