Support for post reactions

[RiccardoM]

Context

In the era of current social networks, more and more applications are allowing users to add more than simple likes to posts. As an example, we've seen Facebook reactions:

Another one is the ability on Slack to react with emojis:

I think we should support this feature too as it make Desmos even more attractive to applications developers.

Post reaction implementation proposal

Currently we represent the like object using the following data structure:

type Like struct {
    Created sdk.Int        `json:"created"` // Block height at which the like was created
    Owner   sdk.AccAddress `json:"owner"`
}

I propose to expand and rename this structure to support other reaction types as follows:

type Reaction struct {
    Created sdk.Int        `json:"created"`
    Owner   sdk.AccAddress `json:"owner"`
    Value   string         `json:"value"`
}

The only change is the new Value field which allows application to insert any kind of text inside the reaction. This might be a simple like string, or even emojis.

Storage changes

No storage changes should be required.