Improve Plasma announcement template

Put the intro of Plasma bugfix announcements into the template, now only actual changes are needed in the post content.

• Time since the previous version by default is inferred from the patch version number. Examples: 5.20.1 to 5.20.5;
• If the real time since the previous version is different from the planned time length, we can go the manual way without introducing new messages in www_www.pot. Example: 5.18.6;
• Support these types of media above the text: youtube, peertube (well, hope it can be back), video (only for 5.19 and 5.20), and figure. For figures, there are default alt and caption values if not defined, works for beta versions as well. Examples: 5.16.90, 5.17.0, 5.19.0, 5.20.1, 5.21.1;
• Compare date of the first minor version with current date to tell whether year is to be displayed in the text. Example: 5.12.9.

Related: sdk/releaseme!9 (merged)