Remove nearly all jQuery from ra.sidescroll.js

[jdufresne]

All DOM methods now use native browser features.

The only remaining jQuery is used to listen to the custom jQuery event
"rails_admin.dom_ready".

Refs #2893

[coveralls]

Pull Request Test Coverage Report for Build 1459412251

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-2.09%) to 93.753%

---

Files with Coverage Reduction	New Missed Lines	%
app/controllers/rails_admin/application_controller.rb	4	91.11%

Totals
Change from base Build 1415561078:	-2.09%
Covered Lines:	3377
Relevant Lines:	3602

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1483588941

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 95.76%

---

Totals
Change from base Build 1474499195:	0.1%
Covered Lines:	3636
Relevant Lines:	3797

---

💛 - Coveralls

[mshibuya]

Thanks!