Improvements in issue template

What

(in the default template)

1. Improves the comments spacing and indentation
2. Adds Summary heading
3. Encloses the OS data in code block
4. Adds a brief description on using code block

(in the project)

5. Uses the directory structure and .md file for template (without it, the template resides in group/project settings)

6. Adds a quick template split to !2 (closed)

Why

• to make the comments look clean and to improve their readability (1.)
  - currently (2020.05.19) the comments' content is mixed with the comment syntax
• to avoid clutter in the title (2.)
  - also see #1649 (comment 331351217)
• to avoid and close #1618 (closed) (3. & 4.)
• since changes to the template from settings can't be suggested via MR as there is no file for that (AFAIK) (5.)
• to make the changes in template easy, transparent and loggable (5.)
• as it is also required for Description Templates (5.)
  - also see #1649 (comment 331357468)

- to save steps of the frequent issue filers/reporters (6.)

Demonstration

Wannna see a live sample and try how it looks? - (click me)

Updates:

comment 348745656 Merged in 637f5671
comment 348667986 Custom templates disabled till we have multiple templates