Add support for sending gifs via Giphy

.editorconfig

sticker/lib/matrix.py

web/index.html

web/lib/htm/preact.js

web/package.json

web/res/giphy-dark.svg

web/res/giphy-light.svg

web/res/powered-by-giphy.png

web/src/giphy.js

web/src/index.js

web/src/search-box.js

web/src/widget-api.js

web/style/index.css

web/style/index.sass

web/yarn.lock