Response image improvements

[jcraigk]

1. Templating. Currently response image content is inserted by splitting on IMG_DELIM constant. This simple mechanism should be replaced with a templating system with open/close tags so gsub-like replacements occur instead of splitting.

2. Storage. Currently we default to STORAGE_PATH being set to /tmp, which must be changed when developing locally. Is there a way to make this dependent on an env var? Any way to construct the image in memory without local storage requirements? Can the cache location be S3? It would be nice to eliminate local storage requirements altogether.

3. Custom content. What would be a good way to allow organizations to incorporate their own images?

4. Display emojis in note echoes.