Additional CMS Pattern Library improvements.

[sabina-talipova]

Description

The following improvements need to be made:

• Replace 4 space indent with a 2 space. ESlint doesn't help.
• Introduce a new library @storybook/addon-docs to present documentation to the components.
• Fix / investigate the reasons for some small warnings that happens time to time in browser console when storybook is running.

AC's

• Component stories have 2 space indent everywhere in the code.
• @storybook/addon-docs is implemented and all stories that have pre-existing readme doc are using it.
• If possible there are not any warnings in the console when storybook is running.

PR's

• ENH Storybook documentation silverstripe-admin#1520
• ENH Storybook documentation silverstripe-asset-admin#1365
• ENH Storybook documentation silverstripe-session-manager#146
• ENH Storybook documentation silverstripe-tagfield#245

[emteknetnz]

@sabina-talipova I've approved all the PRs - you can self-merge them once silverstripe/silverstripe-versioned-admin#296 has been merged - we'd want to merge that one first so that builds go green after merging