When investigating an issue on a customer database, the diff view modal
can be pretty useful. However it is not very good looking and therefore
poorly readable.

This PR improves the CSS styling of that modal so that it resemble more
the diff view of GitHub. This is done by:
- Increasing the width of the modal
- Aligning the text to the top of the table cell, that way there is no
text floating in the middle of two lines
- Lightly coloring the whole line when there is a change on it while the
actual change is on a darker background
- Putting in red all types of change on the left and in green on the
right (instead of mixing green, red and orange together)

This commit is improving the tool that was made at [`96d3fa4e`](https://github.com/odoo/odoo/commit/96d3fa4e01bf8afc35dbf0b7301ce75c6bf3a5c7

)

closes odoo/odoo#127282

X-original-commit: bc188d62
Signed-off-by: Romain Derie (rde) <rde@odoo.com>