Refactor LESS styles to use logical properties

[claudiush]

To drastically improve right-to-left language support I have manually updated all inline (left, right) properties to use their logical equivalent.

Changes proposed in this pull request:
All core LESS styles to use logical properties for inline (left, right) styles. Global browser support today sits above 97%: https://caniuse.com/css-logical-props

Reviewers should focus on:
I did test and click through as many pages, modals and page variants as possible to validate the changes render identically, but more sets of eyes would always help

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• [Does not apply] Frontend changes: tests have been added, or are not appropriate here.
• [Does not apply] Backend changes: tests are green (run composer test).
• [Does not apply] Backend changes: tests have been added, or are not appropriate here.
• [Does not apply] Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite).
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.