Add some missing icons

[RobinMalfait]

• add missing rss icon
• add missing chevron-up-down (formerly known as selector)
• add missing bars-arrow-down (formerly known as sort-descending)
• add missing bars-arrow-up (formerly known as sort-ascending)

[xt0rted]

@RobinMalfait some of the optimized versions don't look right such as bars-arrow-down.svg (the source file looks ok though).

[RobinMalfait]

@xt0rted Thanks for mentioning, I also saw it and working on it 👍