Row selection template container padding

[damyanpetev]

Description

The Row selector templates (both header and per-row) are wrapped in a container w/ .igx-grid__cbx-selection that applies pretty sizable side padding that any user providing a custom template would have to figure out a way around.

• igniteui-angular version: 8.2.x
• browser:

Steps to reproduce

1. Run samples from Add row selectors template samples for all grids igniteui-angular-samples#1368
2. Go to http://localhost:4200/samples/grid/grid-selection-template-numbering
3. Remove overridden style:
   https://github.com/IgniteUI/igniteui-angular-samples/blob/3788c6ea4d7bd828102b7d04027d8b79ea6e8f94/src/app/grid/grid-sample-selection-template-numbers/grid-sample-selection-template-numbers.component.scss#L41-L43
4. Observe the resulting selector column sizes

Result

Selectors are now bumped by padding without the override

Expected result

Shouldn't need to override the padding intended for our default checkboxes template - that should be part of our template and not affect custom ones.