Move nextURL from RouterActionCreator to RouterStore

[misstick]

tasks

• [x] wait for merge of https://github.com/cozy/cozy-emails/pull/898,
• [x] move code from actions to store,

  FIX

• [x] show conversation issue: a click on a messageLisItem trigger JavaScript error and no conversations was displayed,
• [x] messages list issue: unreadMailbox or flaggedMailbox didnt have any messages when the should have some,

  Tests on methods

• [x] getMessagesPerPage
• [x] hasNextPage
• [x] getLastPage
• [x] isPageComplete

[misstick]

@aenario : I didn't have the time to handle these tests:

• At first nothing should be stored
• Each request should be saved with a uniq URI
• Nothing should be stored with falsy params

But all the most complicated is already done into Goto last page should setisCompletefalsy; feel free to finish this in another PR :]]

[m4dz]

Woohooo \o/ !