ui: datePicker - When using starkDateMask, the date is broken when edit it

[SuperITMan]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[X] Bug report  
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

When click on the date to edit it:

• April 23, 2019 becomes 23/01/2019
• When edit 23/01/2019 to 23/04/2019, it becomes April 23, 0019

Expected behavior

The date should not be broken by the date mask.

Minimal reproduction of the problem with instructions

Go in the demo app, in demo-ui > components > date-picker and reproduce the same steps than the one if the video above.

Environment

Stark version: 10.0.0-beta.7

[christophercr]

After investigating the issue, the problem is due to 2 things:

1. wrong behavior of the mask when deleting characters. The mask moves the rest of the characters backwards which causes the date picker to get weird date values. See https://github.com/text-mask/text-mask/blob/master/componentDocumentation.md#keepcharpositions
2. the translation initialization logic is not correct. The locale set in Moment is en-gb but it should be en (US English). This is because the last imported locale sets the current locale (in the Showcase/Starter the last imported locale was en-gb).

Apart from this, we should improve the logic when the application language changes because right now the Moment locale and the Material Date Adapter are not changed, which means that the date pickers don't show the date in the current language :(

[christophercr]

I've created this PR (#1271) for the different fixes

[christophercr]

The issue about the wrong behavior when deleting characters will be partially fixed in #1271 so we can release soon.

However, this should be completely fixed later on. This will be tracked in #1277