[openstreetmap/openstreetmap-website] Add table borders for legibility in richtext diary posts (#2959)

[Michal Migurski]

Introduce new table border treatment for richtext areas in diary entries. In diary entries with tables like [*Calculating label points with PostGIS*](https://www.openstreetmap.org/user/pnorman/diary/394676), [*2020 Editor Usage*](https://www.openstreetmap.org/user/Glassman/diary/394740), and [*Querying OpenStreetMap Changesets with Amazon Athena*](https://www.openstreetmap.org/user/Jennings%20Anderson/diary/394762) table elements appear squashed and hard to read.

This change slightly expands padding within table cell elements and adds thin, light-gray lines between rows and cells to make wider and more complex tables easier to read. Screenshots below are from Firefox 82.

### Current Site

<img width="453" alt="Screen Shot 2020-11-13 at 3 50 45 PM" src="https://user-images.githubusercontent.com/58730/99131442-ff7c7c00-25c7-11eb-91b1-44594009fc2b.png">

### This Change

<img width="453" alt="Screen Shot 2020-11-13 at 3 49 29 PM" src="https://user-images.githubusercontent.com/58730/99131379-d360fb00-25c7-11eb-8747-156a06b68041.png">

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/2959

-- Commit Summary --

  * Added table borders for legibility

-- File Changes --

    M app/assets/stylesheets/common.scss (47)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/2959.patch
https://github.com/openstreetmap/openstreetmap-website/pull/2959.diff

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