Fixed dropdown position for grid and modal

[Blackbaud-SteveBrush]

Addresses: #1440, #1441

~~~See: https://stackoverflow.com/questions/3387489/make-child-visible-outside-an-overflowhidden-parent#comment25311834_10864135~~~

Went with fixed position instead of absolute. The only downside is a slight delay during window scroll. This is similar to Angular Material, where they create an overlay component (with fixed positioning) and inject the autocomplete dropdown into it. On window scroll, the top and left coordinates are recalculated.

https://material.angular.io/components/autocomplete/examples

[codecov-io]

Codecov Report

Merging #1450 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1450   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         370     370           
  Lines        6961    6974   +13     
  Branches      895     896    +1     
======================================
+ Hits         6961    6974   +13

Impacted Files	Coverage Δ
src/modules/popover/popover.directive.ts	100% <ø> (ø)	⬆️
src/modules/dropdown/dropdown.component.ts	100% <100%> (ø)	⬆️
src/modules/popover/popover-adapter.service.ts	100% <100%> (ø)	⬆️
src/modules/popover/popover.component.ts	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f4d21a6...22c8c45. Read the comment docs.