Use 0.15s transition duration for flash close

What does this MR do?

Small UX improvement. Halve the time of the flash close duration.
Additionally add a leading-zero.

Sample

https://jsfiddle.net/8aw0tcp2/

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

Testing is already covered in spec/javascripts/flash_spec.js.
Browsers that already implement transition-duration should have no problem with the difference of timing.