Fix MR Selection, Go Code Refactor (#358)

refactor: Refactors the Go codebase into a more modular and idiomatic approach
fix: require selection of specific MR when there are multiple targets for a given source branch
feat: Allows for the passing of Gitlab's filter options when choosing an MR, improves MR selection
feat: API to choose an MR from a list based on the provided username's involvement as an assignee/reviewer/author

This is a #MINOR release

cmd/app/approve.go

@@ -0,0 +1,50 @@
+package app
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type MergeRequestApprover interface {
+	ApproveMergeRequest(pid interface{}, mr int, opt *gitlab.ApproveMergeRequestOptions, options ...gitlab.RequestOptionFunc) (*gitlab.MergeRequestApprovals, *gitlab.Response, error)
+}
+
+type mergeRequestApproverService struct {
+	data
+	client MergeRequestApprover
+}
+
+/* approveHandler approves a merge request. */
+func (a mergeRequestApproverService) handler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	if r.Method != http.MethodPost {
+		w.Header().Set("Access-Control-Allow-Methods", http.MethodPost)
+		handleError(w, InvalidRequestError{}, "Expected POST", http.StatusMethodNotAllowed)
+		return
+	}
+
+	_, res, err := a.client.ApproveMergeRequest(a.projectInfo.ProjectId, a.projectInfo.MergeId, nil, nil)
+
+	if err != nil {
+		handleError(w, err, "Could not approve merge request", http.StatusInternalServerError)
+		return
+	}
+
+	if res.StatusCode >= 300 {
+		handleError(w, GenericError{endpoint: "/mr/approve"}, "Could not approve merge request", res.StatusCode)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	response := SuccessResponse{
+		Message: "Approved MR",
+		Status:  http.StatusOK,
+	}
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+}