fixed bug DOCS-1677 : Discard message popup opens behind mail compose

It happens in case, there are multiple modal windows at the same time. The overlapping does not happen the right way.

When the positioned".io-ox-dialog-wrapper" - modal wrapper element pops up, it is on the same level as another positioned elements with z-index set to some fixed value. Because of the fact that ".io-ox-dialog-wrapper" - element does not have a fixed z-index value, no new local stacking context is formed, but it's children do have it - so they establish a new stacking context and go to the same stacking context with the siblings of their parent - ".io-ox-dialog-wrapper" element. And that leads to the behavior - the children's z-index values are being compared with the z-index values of their parent's - ".io-ox-dialog-wrapper" element, and not their parent's z-index value itself.

• set "z-index: 14;" - on the ".io-ox-dialog-wrapper" modal window wrapper element. That means, that ".io-ox-dialog-wrapper" forms a new stacking context and is compared with that of it's siblings, regardless of z-indexes of it's children. z-index should be set at least to 14 - the highest value of it's sibling elements.