we do not want our own i18n database.
When this kind of data is needed intl extension is a requirement.

- removed unformat methods, they do not belong here
- removed db format, which is also not purpose of this class
- refactored the whole set of methods to be simpler and better
  maintainable

More unit tests needed.

Merge branch '2359-formatter-refactored' of https://github.com/Erik-r/yii2-1 into Erik-r-2359-formatter-refactored

* '2359-formatter-refactored' of https://github.com/Erik-r/yii2-1:
  #2359 Bugfix in normalizeDatetimeValue after regression test
  #2359 one test failed in Travis because standard medium date format is different in Travis then on my locale PC.
  #2359 testcases adapted and compatibility to old tests improved
  remove comment lines in asRelativeTime
  Typo in function call #2359
  Namespace corrected #2359
  Refactored formatter class #2359 which works with or without intl extension. Use PHP format patterns alsow with intl. Class is compatible with previous version.

[ci skip]

intro upgrade from v1 guide translated

Create caching-page.md

French start-installation.md

Fix typo in start-installation.md

fixes #3891

Translation es

English guide small fix [skip ci]

Translate "Page Caching"

docs about file validator fixed

Spanish guide polish and start-looking-head.md [skip ci]

to be adjusted while translating chapters.