[FIX] Trade filter on history page

Trade filter shows only partially/fully funded trades. I would call this a feature, but it's classified as a bug in Jira, hence the [FIX] label.

[Jira RT-1835](https://ripplelabs.atlassian.net/browse/RT-1835)
[Bountysource](https://www.bountysource.com/issues/2842682-create-trade-filter-in-history-tab)

(- modified since last pull request. "Orders" filter is now identical to earlier "offers" filter.)
[FIX] Unfunded banner flicker (RT-2064)
[FIX] XAU/demmurage not working when flipping or clicking on an order
[FIX] Currency pairs not updated in drop after flipping/clicking on an order
[FIX] Invalid/non-canonical currency pairs saved in drop menu

Fixs a bug in RT-1876 which will leave invalid currency pairs in the drop menu.
Now I use Currency#to_json() to generate canonical names for currency pairs.
Old non-canonical currency pairs will be canonicalized too.
i.e. "xaU - (-0.5%pa)/cNy" will be canonicalized into "XAU (-0.5%pa)/CNY".
Merge pull request #1386 from Madsn/patch-unfunded-banner-rt-2064

[FIX] Unfunded banner flicker (RT-2064)
Merge pull request #1376 from Madsn/trade-filter

[FIX] Trade filter on history page
[FIX] Jsonrewriter: Don't calculate order price for zero amounts
Merge pull request #1390 from vhpoet/accountbalance

[FIX] Jsonrewriter: Don't calculate order price for zero amounts
Merge pull request #1388 from orzFly/patch-demmurage-trade

[FIX] Problems about demmurage, flipping and clicking on order
[FEATURE] In Send/Convert, user should be able to specify the gateway (RT-1790)