Begin of the insertionThe new date picker will get some style updates mostly with regards of structure and re-use of existing Design System components (text-field, buttons etc.

**Acceptance (images copied from the feature ticket)**

* The modal has a new structure (banner - scheduling mode - dates - calendar - action bar)
* The date picker for Milestones slightly differs as there is only one date field
* There are some styling changes (Design system conform)
* The buttons and text fields have our new styles
* The banner can be in two different colours (or not existent at all)
* The calendar selection style is updated and can be in state "disabled" with greyed-out style

<figure class="image op-uc-figure" style="width:50%;"><div class="op-uc-figure--content"><img class="op-uc-image" src="/api/v3/attachments/31094/content"></div></figure>

* The mobile version looks like this

<figure class="image op-uc-figure" style="width:50%;"><div class="op-uc-figure--content"><img class="op-uc-image" src="/api/v3/attachments/31588/content"></div></figure>

**Out of scope**

* Any kind of behavioural changes including:
* Real content for the banner
* Correct selection of dates
* Correct display of datesEnd of the insertion