fix(GlTable): do not override CSS utils styling

This rewrites some of `GlTable`'s styles to not override CSS utils
styling.

BREAKING CHANGE: This rewrites how `GlTable`'s `TH` elements text
alignment is applied. If you were previously passing `gl-text-right` to
a table's `thClass` field option, you should migrate to the
`thAlignRight` option instead. Not migrating could cause rendering
issues in right-aligned sortable columns.

Before:

```js
const fields = [
  {
    key: "column_one",
    label: __("First column"),
    sortable: true,
    thClass: 'gl-text-right',
  },
];
```

After:

```js
const fields = [
  {
    key: "column_one",
    label: __("First column"),
    sortable: true,
    thAlignRight: true,
  },
];
```