Update sizes used in the loading icon (spinner) component

Problem

Currently, the loading icon (spinner) component implemented across the product is using deprecated sizes (1, 2, 3, 4, and 5).

Copying some notes from gitlab-ui!614 (merged):

Currently, the most used size, across 119 component instances, is 2 and md which will be changed to lg once gitlab-ui#308 (closed) gets implemented. The rest instances are using the default size which was md, right?

Size	How many instances?
1	0
2	20
3	9
4	1
5	0
sm	6
md	15
lg	0

Proposal

1. Replace all sizes from 2 to lg.
2. Replace all sizes from 2 also to lg.
3. Replace all sizes from 4 to xl.
4. Change all md sizes to lg.

Risks

Just have to verify step 2 is not affecting negatively the UX.

Replace all sizes from 2 also to lg.

Before closing / Next steps

Once the steps above have been completed, proceed with opening a follow-up issue to remove support for legacy sizes. See relevant discussion in gitlab-ui!633 (comment 195902770).

See also additional notes in loading_icon.md and specs to be removed in loading_icon.spec.js