Make collapsible sections infinitely nested

What does this MR do?

This makes the collapsible sections of the job log feature infinitely nested, addressing some of the current bugs and improving the overall performance of the collapsible section algorithm.

The changes are behind the following feature flag: infinitely_collapsible_sections

Rollout issue for the feature flag: #335297 (closed)

Issue for the feature: #29766

Screenshots or Screencasts (strongly suggested)

Multiple inner collapsible sections

Screen_Recording_2021-07-06_at_10.17.34

An example CI config file to try the above is:

Click to expand

stages:
  - qa

image: alpine

collapsible_job_single:
  stage: qa
  script:
    - echo -e "\e[0Ksection_start:`date +%s`:my_first_section\r\e[0KHeader of the 1st collapsible section"
    - echo 'this line should be hidden when collapsed'
    - echo -e "\e[0Ksection_end:`date +%s`:my_first_section\r\e[0K"

collapsible_job_multiple:
  stage: qa
  script:
    - echo -e "\e[0Ksection_start:`date +%s`:my_first_section\r\e[0KHeader of the 1st collapsible section"
    - echo 'this line should be hidden when collapsed'
    - echo -e "\e[0Ksection_start:`date +%s`:second_section\r\e[0KHeader of the 2nd collapsible section"
    - echo 'this line should be hidden when collapsed'
    - echo -e "\e[0Ksection_end:`date +%s`:second_section\r\e[0K"
    - echo -e "\e[0Ksection_end:`date +%s`:my_first_section\r\e[0K"

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.