[Discover] Improve numeric column text wrapping and some minor touchups

[davismcphee]

Summary

This PR includes a couple of minor improvements to the Unified Data Table multiline header implementation:

• Improves multiline wrapping for numeric columns so that text wraps from the left instead of right.
• Removes the need for manual line-height adjustment on field tokens by adding vertical-align: bottom (or top, either works in this case) to the FieldIcon itself.
• Replaces a couple of magic numbers in the CSS with equivalent EUI theme variables.

resize.mp4

[mbondyra]

tested against my PR and I love the improvements! Thanks!