Update issues slice to index issues instead of list them on an array

[leandrooriente]

The current implementation has an array that is cleared and repopulated every time a new list of issues comes.

We can optimize so the user won't have to load a page that was already loaded and opening an issue will only require fetching its comments.

Eg:

const store = {
  page: 1,
  pageCount: 2,
  issuesPerPage: {
    1: ['1231765'],
    2: ['898232']
  },
  issues: {
    1231765: {
      title: "Foo Issue",
      labels: []
    }, {
    898232: {
      title: "Bar Issue",
      labels: []
    }
  }
}